This is a proposal to continue a systematic molecular genetic investigation of Drosophila melanogaster indirect flight muscles (IFM), our goals being to elucidate the mechanisms by which myofibrils are assembled, maintained, and generate regulated force and tension. During previous grant periods we characterized genes that encode Drosophila actins, tropomyosins, troponins, and alpha-actinins, and analyzed muscle abnormalities engendered by their mutant alleles. This work significantly improved our understanding of the genetics and biochemistry of contractile proteins and provided insights into fundamental aspects of myofibril assembly and function. During the next five years, we propose to continue investigating substantive issues of muscle and cytoskeletal molecular biology. The core of our project will continue to be mutant analysis and our principal aim will be to generate an informative series of actin mutations. Flies having these alleles should be interesting subjects for both structural analyses and motility assays. We will also continue our studies of Drosophila troponins. We intend to characterize three previously identified troponin-T and -I mutants whose morphologically normal flight muscles disassemble upon activation. We will examine the expression of troponin-C isoforms and correlate particular genes with extant mutant alleles. Finally, we will conduct three projects to elaborate how thick and thin filaments are integrated into the myofibril lattice. In the first, we will establish whether interactions of thick and thin filaments are required for polymerization to precise lengths. To address this issue we will measure filaments in mutants lacking sarcomeric actin or myosin. In the second we will examine whether myofibrils are templated or nucleated by cytoskeletal elements. Nascent myofibrils will be examined in mutants lacking sarcomeric actin, and we will use mutations to perturb cytoskeletal elements of muscle. Finally, we will employ conditional alleles of actin and flightless genes to perturb thin filaments during intervals of muscle development, and subsequently assess the effects. Results of our investigation are expected to improve our understanding of muscle cell biology and illuminate the molecular bases of muscle dysfunction.